1. Field of the Invention
The present invention relates to a frequency band control device and, more particularly, to a frequency band control device advantageously applicable to controlling a frequency band of a line constituting an ATM (Asynchronous Transfer Mode) network.
2. Description of the Background Art
Today, there is an increasing demand for a B-ISDN (Broadband Integrated Services Digital Network) capable of implementing high speed, high quality communication. Standardization of ATM technology which is the heart of the B-ISDN is progressing at high speed. The ATM technology divides data to be transmitted into segments of forty-eight bytes, adds five-bytes of control information or a five-byte header to each forty-eight bytes so as to form a cell having a fixed length of fifty-three bytes, and transfers such cells via a transmission path. Because the ATM technology is feasible for high speed switching using hardware, it is attracting increasing attention in the B-ISDN field, which is required to transfer a plurality of media information lying in different frequency bands at the same time.
When a single line is shared by a plurality of channels, the conventional ATM technology fixedly allocates a frequency band available with the line in accordance with the frequency band needed by the individual channel. For this kind of control, it is a common practice to use a table listing information for designating the channel to occupy the line on a time slot or cell time basis. The position of the table from which the information is to be read is sequentially shifted every cell time, so that the frequency band is allocated to the channels.
The above frequency band control device, however, has the following problem left unsolved. Assume that a buffer assigned to the channel designated by the table stores no cells to be sent to the circuit. Then, not only the above channel is wastefully designated, but also cells cannot be sent from the other channels on which cells to be sent exist, resulting in an unoccupied frequency band.